Recently, portable terminal equipment such as cellular phones and portable information terminals has become widely used due to the development of mobile communications systems. For example, multi-band cellular phones having two or more communication systems have been developed. Portable devices such as multi-band cellular phones are required to have a wideband filter capable of handing the bands of the two or more communication systems.
However, it is difficult to realize a filter that covers the two or more bands and low insertion loss. Therefore, an alternative filter is used in which multiple acoustic wave filters are connected in parallel and each of the filters covers the communication band of the respective communication system.
Japanese Patent Application Publication No. 11-68512 discloses a one-input two-unbalanced-output filter using two acoustic wave filters. Japanese Patent No. 3480445 discloses a one-input two-balanced-output filter using two acoustic wave filters. In the two acoustic wave filters connected to one input in parallel, these acoustic wave filters allow signals in the respective pass bands to pass therethrough. In order to cut off signals out of the pass bands, one of the filters is in a high-impedance state in the pass band of the other filter.
In the above-mentioned prior art documents, a series resonator is provided between the particular acoustic wave filter and the input in order to adjust the impedance so that one of the filters is in the high-impedance state in the pass band of the other filter.
However, the insertion of the series resonator results in insertion loss and prevents downsizing.